1. Field of the Invention
The present invention relates to an optical disc substrate consisting of a disc supporting portion formed around the center thereof and of which one side provides a disc supporting surface, and a recording portion formed between the disc supporting portion and a circumference thereof and of which one side provides a recording surface, and an optical disc incorporating this optical disc substrate. More particularly, it concerns with an optical disc substrate in which the thickness of the disc supporting and recording portions can be determined to attain a high density of recording while maintaining a sufficient rigidity without easy warping of the substrate, and an optical disc incorporating the optical disc substrate.
2. Description of Related Art
These days, researches and developments of various optical information recording systems have been made everywhere in the field of information recording. The optical information recording system is advantageous in many respects. For instance, information can be recorded and reproduced with no contact with a read/write head, information can be recorded with a density higher more than one order of magnitude than the conventional magnetic recording system, and there are available various types of media, such as read-only, recordable and rewritable. As a recording system enabling an inexpensive, large-capacity filing, many application are expected of the optical information recording system to meet industrial needs and home or personal demands.
Among others, the compact discs (CD) and optical video discs (DVD), of the read-only type, have been widely used.
More specifically, a CD comprises an optical disc substrate being a transparent one having formed thereon a pattern of convexities and concavities such as pits and grooves indicative of information, a reflective layer made of a metal film such as aluminum film, and a protective layer formed on the reflective layer to protect the latter against moisture and O.sub.2 in the atmosphere. To reproduce or read information from such an optical disc, a reading light such as laser light is irradiated onto the convexity and concavity pattern from the optical disc substrate side, and the information is detected based on a difference in reflectivity between the incident and return lights.
Recently, a higher density of information recording has been required. To meet this demand, it has been proposed, in a tendency that a light of a shorter wavelength is adopted in reading/writing of optical discs, to use a means for increasing the numerical aperture (NA) of an objective lens provided in an optical pick-up and through which a reading light of a certain wavelength is irradiated to an optical disc, to decrease the light spot diameter. For instance, the NA of objective lens used in an optical disc drive for CD is 0.46, but that of an objective lens used in a player for DVD having recently been catching people's attention for its recording capacity 6 to 8 times higher than CD, is about 0.60.
As the NA of the objective lens is larger as mentioned above, an optical disc substrate through which an irradiated reading light is transmitted should have an increased thickness.
That is, with an objective lens having a larger NA, a larger astigmatic aberration will be caused by a tilt of an optical disc surface with respect to an optical axis of an optical pick-up and thus the tilt angle allowance for the disc surface will be smaller. Note that the tilt angle for CD is 0.6.degree. or less while that for DVD is 0.4.degree. or less.
The allowance of the tilt angle can be determined from a relation 1/(optical disc substrate thickness).times.NA.sup.3, and thus it will easily be affected by the thickness of an optical disc substrate and NA of an objective lens. Namely, when an objective lens having a larger NA is used, the thickness of optical disc substrate should be smaller for a maximum possible allowance of tilt angle.
To meet such requirements, the optical disc substrate for CD is on the order of 1.2 mm while that for DVD is about 0.6 mm.
However, an optical disc substrate having a reduced thickness will have a correspondingly decreased rigidity. Generally, a plate has a flexural rigidity proportional to the third power of its thickness. Therefore, when a plate has the thickness reduced to 1/2of its original one, its rigidity will be 1/8of the original one. Thus, for DVD to keep a sufficient rigidity, they are made of two laminated optical disc substrates attached to each other.
However, to assure a sufficient rigidity of the optical disc by such a method, a process of lamination not required for manufacturing an optical disc substrate usable as a single-plate structure will add to the processes of optical disc substrate production, whereby the productivity will be lower. When producing an optical disc having a double-plate structure in which one of the substrates has a recording layer while the other substrate is a dummy one to reinforce the rigidity of the optical disc, the manufacturing costs will be increased while the productivity be lowered.
As discussed in the above, an optical disc substrate for use in magneto-optic recording using a magnetic field modulation should be of a single-plate structure. The above-mentioned laminated optical disc substrate cannot be used for magneto-optic recording based on the magnetic field modulation.
In these circumstances, an optical disc using a single thin substrate has been demanded increasingly more.
However, when an optical disc using a single thin substrate is played with a DVD-oriented disk driving means to spin the optical disc, the disk driving means cannot catch the optical disc by in the direction of the disc thickness a chucking mechanism thereof. Thus, such an optical disc is not compatible with any other disk driving means than dedicated for the optical disc itself. It will be necessary to provide a new disk driving means for such an optical disc, which however will disadvantageously lead to an increased running cost.